During the treatment of liquid metals, and more particularly of steel and cast iron, certain metallurgical operations call for stirring of the molten mass. This stirring may be effected in various ways, for example, by an electromagnetic effect or by the introduction of gaseous or liquid fluids through nozzles or permeable refractory parts.
The method presently employed the most consists in blowing a gas through permeable refractory parts placed in the refractory lining of the container, preferably in the bottom. In order to avoid leakage the walls of the bottom of the permeable parts are rendered gastight by various means such as the presence of a dense refractory layer integral with the permeable refractory, or a layer of metal deposited on the permeable refractory, or more usually by a sheetsteel envelope.
During use the permeable parts undergo numerous attacks: besides the usual strains to which the whole lining is subjected, the permeable refractory must withstand thermal shocks at the time of the flow of gas, the eroding action of the bath in motion and the cleaning after the emptying of the container.
Consequently, the permeable parts wear very rapidly and do so in spite of the use of highly refractory materials such as magnesia or alumina. What follows is that the length of life of the permeable parts is in general less than that of the rest of the lining. Thus, these parts are considered the weak points in the lining, the more so because they often wear in an irregular and unforeseeable manner.
These permeable parts are generally made of sintered refractory materials based on alumina or magnesia. The disadvantage of these parts lies in their poor behavior under the abrupt variations in temperature which take place every time blowing in is carried out anew. Thus it is often found that the sintered permeable refractory parts flake off and disappear into the slag.
This disadvantage may be overcome by replacing the sintered refractories by parts of permeable refractory concrete, generally having a base of sintered or electrically fused alumina and of superaluminous cement of "Secar" 250 type.
These parts withstand the variations in temperature distinctly better than the sintered qualities but they have another deficiency. In the temperature zone lying between 300.degree. and 900.degree. C. aluminous refractory concretes become very friable for lack of ceramic bonding. Their compressive strength drops in places as low as 20 kg/cm.sup.2 and even lower.
In certain cases the mechanical erosion of the concrete which has not had time to sinter is very great and consequently shows as a still more rapid wear of the permeable parts than with sintered porous refractory materials.